Elements relating to the performance of color toners include color strength and transmittance. Although the color strength and the transmittance are affected by the amount of a colorant, these properties are also greatly affected by the dispersibility of the colorant in the color toner particles. In a process for manufacturing a color toner by pulverization method, the dispersion states of the various raw materials in the color toner particles are substantially determined by the steps of mixing raw materials, and kneading the mixture.
In order to solve the above problem, Japanese Patent Laid-Open No. Hei 6-161154 discloses a technique for dispersing a water-insoluble dye and/or pigment in a resin binder by flushing method. In addition, Japanese Patent Laid-Open No. Hei 6-11895 discloses a technique of a melt-kneading process divided into two steps, comprising granulating a raw material mixture in the first kneading step; and adding additives such as a resin binder to dilute and mix the mixture in the second kneading step. However, in these attempts, there also arise some problems that specialized plurality of steps are necessitated, and that the number of steps are very large, thereby making the process complicated.
In addition, in a process using an extruder as that described in Japanese Patent Gazette No. 2677685, there arises a problem that sufficient dispersion of a colorant, a charge control agent, a wax, and the like required for a color toner cannot be carried out as the temperature of the composition increases by kneading heat.
Also, there are numerous prior arts disclosing that a wax is used as an offset inhibitor by blending it in the toner. However, as the amount of the wax blended increases, the wax in the toner is poorly dispersed, so that the wax deposits on the photoconductor in the developer device, and that the level of the triboelectric charges is lowered with the passage of time, thereby making it less likely to carry out stable development.
In order to solve these problems, Japanese Patent Laid-Open No. Hei 6-161153 discloses a technique of kneading the mixture at a set temperature (Ts), wherein the set temperature is within the range of Tm-20.degree. C..ltoreq.Ts.ltoreq.Tm+20.degree. C., wherein Tm is a melting temperature of the resin binder. However, in order to sufficiently disperse the wax blended into the toner, it is necessary to apply a strong kneading force. When a twin-screw kneader described in Japanese Patent Laid-Open No. Hei 6-161153, since the temperature of the kneaded mixture increases by 20.degree. to 60.degree. C. or so from the set temperature owing the generated kneading heat, the wax is re-agglomerated, and a strong kneading force cannot be applied, so that there arises a defect that a toner having excellent dispersibility of the wax cannot be obtained. Especially, this phenomenon is markedly found when a wax having a low melting point (83.degree. C.), such as carnauba wax, is used. On the other hand, when the set temperature is lowered in consideration of the kneading heat, there arises a defect that the resin does not melt, so that kneading cannot be carried out.